Maybe it's because it was the first one I played, but something about the 2d aspect of this game gripped me. I don't have the same love for the games that came after it, the 3d graphics somehow take me out of the historical element.
The fact that in the older games you are basically playing on a map, gives off this almost board-game feeling that is cosy and immersive. (I know that civ was inspired by a board game)
Playing civ3 as a kid really made me feel like I was in whatever era I was playing at. Still remember those jazz fusion tracks especially.
Civ III is not one of my favorites, but my best memory of Civ III is probably the game where I played a game on emperor difficulty and one AI side was neck and neck with me and we spent well over two centuries having an air war of mostly bombers. It was honestly ridiculous how closely matched we were and the war lasted until the end of the game.